Method and Apparatus for Contacting Bubbles and Particles in a Flotation Separation System

ABSTRACT

A flotation separation apparatus for separating particles in suspensions, feeds slurry containing the particles through an inlet ( 10 ) into a contactor ( 16 ) where gas is fed through an inlet ( 12 ) to mix with the slurry, for example in a downwardly plunging jet ( 14 ), to form a gas-liquid bubbly two-phase mixture under pressure from an outlet restriction in a throttling duct ( 18 ). The mixture is passed through a flow manipulator configured to induce a high energy dissipation rate, for example by way of a Shockwave formed in a diverging section of the throttling duct ( 18 ) reducing the size of the bubbles and brining those bubbles into intimate contact with particles in the mixture which is released into a separation cell ( 21 ) where a flow manipulating draft tube ( 20 ) is provided to reduce turbulence in the mixture. Alternative apparatus and methods for inducing the high energy dissipation rate and for reducing turbulence in the mixture are also described and claimed.

FIELD OF THE INVENTION

This invention relates to the froth flotation process for the recoveryor separation of particles from suspensions in liquids in general, andmore particularly to an efficient contacting apparatus and process foruse in flotation separation systems.

BACKGROUND OF THE INVENTION

The flotation process is used in the separation of particles frommixtures in a finely divided state, suspended in a liquid. For example,in the minerals industry, a suspension of solid particles in water istreated with chemical reagents or collectors which have the effect ofmaking the particles which it is desired to remove, water repellent orhydrophobic, while leaving the remaining particles in a wetted orhydrophilic state. The liquid is fed into a flotation separation cell,which may be in the form of a tank or column, and air is injected in theform of fine bubbles. The hydrophobic particles attach to the airbubbles and rise to the surface of the cell, from which they can beremoved by flowing over a lip under the action of gravity, into alaunder or channel. The particles which are not collected by the bubblesremain in the suspension and flow out of the bottom of the cell, in thetailings. Frother reagents are often added to the feed liquid in orderto assist in the formation of a stable froth on top of the liquid in thecell. Clean water may be applied to the froth layer in order to washentrained particles downwards into the cell.

Flotation is also used generally for the recovery of fine particles fromsuspensions in liquids, as in the removal of printing ink from recycledpaper; for the removal of particles especially fat and oil droplets fromwaste waters in the food industry; for removal of particulates inprocesses for the remediation of contaminated sites; for the treatmentof produced water emanating from oil fields; and for the recovery ofalgae and other organisms from suspensions in fresh water or sea water.For purposes of description, the term ‘air’ may be used to represent thegas, ‘water’ may be used to represent the liquid and the floatablecomponent may be referred to as ‘particles’ or in some cases as the‘values’. The non-floating component is referred to as ‘gangue’. It isto be understood however that the same principles apply in other systemsinvolving fine particles that are not minerals, dispersed in aqueous ornon-aqueous media, being floated with gases other than air.

In earlier technology, flotation has been carried out in mechanicalcells in which the liquid is agitated by a rotating impeller and air isintroduced in the vicinity of the impeller. The bubble sizes produced inthese devices are not necessarily small, being typically in the range 1to 5 mm in diameter. More recently, flotation has come to be carried outin columns, which have operational advantages in being able to providebetter control of the phenomena in the froth. Flotation columns incurrent use, vary in the aspect ratio. Some are tall relative to theirdiameter or breadth, with a height-to-diameter ratio of at least 2:1 andup to 10:1 or greater. In these devices the feed slurry is typicallyinjected towards the top of the column, and a stream of bubbles iscreated by a suitable means such as a sparger, injector, aspirator,nozzle or bubble generator. The objective of these aeration devices isto distribute the bubbles essentially uniformly across the cross-sectionof the column. Thus as the stream of particle-laden liquid descends downthe column, it meets a distributed cloud of small bubbles risingvertically. The individual bubbles collide with and capture thehydrophobic values, and carry them upwards into the froth.

Any discussion of the prior art throughout the specification should inno way be considered as an admission that such prior art is widely knownor forms part of common general knowledge in the field.

In both mechanical cells and columns, the contact between bubbles andparticles usually takes place in the liquid in the vessel itself. Thusthe reason for the height of tall column cells, is to provide sufficienttime for the bubbles to come into contact with particles as they rise inthe column. Flotation column cells as described particularly by Finchand Dobby (Column Flotation, Pergamon Press, Oxford, England, 1990),consist of three zones: the froth zone at the very top of the column,typically 1 m in height; the collection zone, where bubble-particlecontact occurs, typically 5 to 10 m in height; and the disengagementzone in the base of the column, where the liquid flows out of thecolumn, typically 1 to 2 m in height. Thus the overall height of acolumn cell is in the range 7 to 13 m. The froth zone must be ofsufficient height to allow the gangue particles to drain, and clean washwater is often distributed over the top of the froth or within thefroth, to wash the gangue back into the liquid in the flotation cell.The disengagement zone is a quiescent location, where the downwardvelocity of the liquid is less than the rise velocity of the bubbleswhich have been introduced higher in the cell, so that the bubbles areable to escape from the exit stream from the column.

Internal bubble generators are known for flotation columns. Some consistof simple distributor pipes with small holes in the walls, or withporous walls. In others, such as the generator of Harach, U.S. Pat. No.4,911,826, an array of fine nozzles is supported by distributor pipesacross the whole cross-section of a tall column. Air and water streamsare supplied through headers, and a mixture of air and water isdischarged through each fine nozzle. In yet others, air under pressureis supplied to tubes made of an elastic material like rubber. Thesurface of the elastic tubes is pierced with an array of very fine holeswhich remain closed when the external pressure is greater than thepressure within the tube. As the internal pressure is increased, theelastic wall stretches and the fine holes enlarge sufficiently to allowthe passage of air, which is discharged from the holes in the form offine air bubbles.

External bubble generators are also known in the tall flotation columncells. Hollingsworth, U.S. Pat. No. 3,371,779, describes a venturi-typeaspirator to produce air bubbles into a stream of fresh water which isthen introduced into the bottom of a flotation column. Christopherson,U.S. Pat. No. 4,617,113, described how a multitude of venturi aeratorscan be distributed around a large column. Air is inspired into waterflowing through the venturis. In the apparatus of McKay and Foot, U.S.Pat. No. 4,752,383, air and water are pre-mixed at high pressures in achamber containing beads. The aerated water is then injected into thebase of a flotation column through a lance, which has a small orifice atthe end. Bacon, U.S. Pat. No. 4,472,271, produced bubbles in slurrytaken from the bottom of the flotation cell. The bubbles were made bypassing air and slurry through a nozzle. The bubble-laden slurry streamwas reintroduced through the wall of the flotation column. Yoon, U.S.Pat. No. 5,397,001, has described a flotation column in which the air isdispersed into slurry in external static mixers. Slurry is taken out ofthe bottom of the flotation cell and distributed equally among a numberof static bubble generators where air is added. The aerated slurrystream is then injected into the flotation column above the externalaerators. In the aforementioned devices, the external devices areessentially bubble generators and contact takes place within the column.

Short columns are known, in which the height and diameter are of thesame order of magnitude, and the height-diameter ratio in industrialapplications may be from 0.2 to 1, to 2 to 1. In these short columns,air is introduced into the feed liquid in an aeration system prior toinjection into the column, and it is in this aeration system thatcontact between bubbles and particles is established. Relatively littlecontact is effected in the column proper. The aeration system may takethe form of a plunging jet, a venturi, a static mixer, or a sparger orporous-walled pipe through which air is introduced in a turbulentfashion into the feed slurry. Examples of such devices are described byJameson, U.S. Pat. No. 4,938,865; and U.S. Pat. No. 5,332,100; Bahr,Ger. Pat. No. 2,420,482; Imhof, Europ. Pat. No. 1,084,753, and Ludke,U.S. Pat. No. 4,448,681. Because of the high-efficiency contacting inthe aeration device, the functions required in the flotation column ortank are much reduced. Thus in principle, there is no need for thecollection zone as found in tall column cells, because bubbles andparticles have already contacted each other. However, the froth anddisengagement zones are required. For present purposes, short flotationcolumn cells of the types described by Jameson and Bahr will be referredto as “intensive” cells. Because there is no need for the collectionzone, the intensive cells have significant advantages over the tallcolumn cells, emanating from the much reduced size.

All of the aforementioned inventions describe processes to disperse airbubbles into a liquid which may or not contain suspended particles.However, none of these bubble-generating devices place any form of flowrestriction that can be used to control or influence the pressure in theair-liquid mixture after formation. It can be advantageous to controlthe pressure at which the bubbles are formed, both in absolute terms andalso in terms relative to the pressure at which they are to be used inthe flotation vessel. For example, when bubbles are generated by thebreakup of a supply of air in a shear flow such as exists in the throatof a venturi, or in a static mixer, the size of the resulting bubbles isa function of the local void fraction, which is the ratio of the volumeof gas under local pressure conditions, to the total volume of gas andliquid. It is generally desirable to minimize coalescence of bubblesafter formation, because it is well known that the rate of capture ofparticles by bubbles diminishes as the bubble size increases, for aconstant air/liquid ratio. Bubble swarms that are created in agas-liquid mixture of low void fraction, are generally more stable,because the rate of coalescence of bubbles is related to the meandistance between the bubbles, which in turn is related to the voidfraction. For the same mass ratio of gas to liquid, the volume ratiovaries inversely as the absolute pressure. Thus if it is desired tosupply a feed liquid with an equal volume of air at the absolutepressure in the flotation cell, it will be advantageous to create thebubbles at a higher pressure than exists in the cell. For example, ifthe absolute pressure at which bubbles are generated is twice theabsolute pressure in the cell, the volume fraction will be one half thatin the cell.

This effect was recognised by Amelunxen (CA Patent Specification2106925), who described an external contactor, a throttle valve forcontrolling the process pressure within the contactor and a system forinjecting air and liquid into the contactor under pressure.

All of the prior art contactors suffer from disadvantages, which canvariously relate to: limitations in the amount of air that can besupplied relative to the amount of liquid flowing through the sparger oraeration device; the necessity for small orifices or tubes which readilycorrode or become blocked by the particles present in the feed; thenecessity for complex and expensive manufacturing processes to provideparts that can withstand the wear associated by high velocity flows; thedifficulty of replacing crucial wearing parts in an operating plant; theneed for relatively high concentrations of frother or other expensivesurface active agent in order to produce small bubbles; high operatingcosts associated with excessive driving pressures in the liquid and/orthe air streams.

There is a range of particle sizes in the feed suspension for whichcurrent flotation technologies are efficient. Thus in an intermediateparticle size range, between 40 and 150 microns for minerals (and 75 and350 microns for coal), conventional flotation cells can achieve highrecoveries. However, when the size of the particles is less than orgreater than the intermediate range, the flotation recovery tends todecrease as the particles become smaller (or larger). For presentpurposes, “fine” particles are those whose diameter is smaller than theappropriate intermediate size range, i.e. those between 0 and 40 micronsfor minerals, and 0 and 75 microns for coal; “ultrafine” particles arethose at the lower end of the “fine” range; and “coarse” particles arethose whose diameter is greater than 150 microns for minerals, and 350microns for coal.

The inventor of the present invention has found that improved flotationof fine particles can be achieved by reducing the bubble size,increasing the gas supply rate relative to the flow rate of particles,and increasing the shear intensity or energy dissipation rate in oradjacent the contacting device. The rate of recovery is related to therate at which the particles collide with the bubbles. Since the inertiaof the particles varies inversely as the cube of the diameter, as theparticles become smaller, so finer particles tend to follow the fluidstreamlines around the bubbles and the probability of attachment isreduced as the size decreases. The recovery of fine particles can beimproved by using smaller bubbles and by increasing the rate of shear inthe contacting system (N Ahmed and G J Jameson, “The effect of bubblesize on the rate of flotation of fine particles”, Int. J. MineralProcessing, 14, (1985), 195-215.). A substantial improvement in theperformance of a typical flotation machine can be expected if the bubblesize is reduced. Accordingly, it has been recognised by the inventorthat for high-efficiency flotation a source of fine bubbles, typicallyin the range 400 microns in diameter or smaller, be provided, in ahigh-energy dissipation rate environment.

For coarse particles, the reduction in recovery as the particle sizeincreases is due to the inability of bubbles and hydrophobic particlesto stay in contact with each other in a highly-turbulent environment.The bubbles tend to move to the centre of vortices or eddies in theflotation cell and the particles are flung away from the bubbles bycentrifugal forces. High recoveries of coarse particles are favoured bya high gas fraction in the slurry suspension, by low levels ofturbulence in the region below the froth layer. It is also favourable toprovide a means to levitate the coarse particles so that their upwardspassage towards the froth is assisted by an upwards motion of liquid inthe region beneath the froth.

It is the purpose of the present invention to provide simple, efficientand economic means to overcome the difficulties and inefficiencies inknown flotation technologies, by generating fine bubbles and bringingthem into contact with the particles to be floated, and controlling theresulting gas-solid-liquid mixture so as to maximise the transfer ofhydrophobic particles into the froth and hence into the flotationproduct.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides an apparatus forcontacting bubbles and particles in a flotation separation system, saidapparatus including;

a contactor arranged to receive under pressure a supply of feed slurryincorporating particles suspended in a liquid and a supply of gas, thecontactor being arranged to mix the slurry with the air forming agas-liquid bubbly two-phase mixture;

an outlet from the contactor configured to provide a restriction to theflow of mixture therethrough and maintain the mixture within thecontactor under pressure;

a flow manipulator downstream from the outlet configured to induce ahigh energy dissipation rate within the mixture passing therethrough;and

a separation cell arranged to receive mixture from the flow manipulatorand allow bubbles with attached particles to rise to the surface ofliquid within the cell.

Preferably the separation cell is provided with a mixture directingdevice arranged to receive the mixture from the flow manipulator andcontrol the release of that mixture into the cell.

Preferably the contactor includes a substantially vertical columnarranged to receive the feed slurry under pressure into the top of thecolumn.

Preferably the contactor incorporates mixing means including a nozzlearranged to form a downwardly plunging jet of feed slurry within thecolumn, and a gas inlet in the vicinity of the jet so formed such thatin use gas is entrained into the jet forming said gas-liquid bubblytwo-phase mixture.

Preferably the outlet from the contactor is configured to form at leastone throttling duct providing said restriction to the flow of mixturetherethrough.

Preferably the throttling duct has a converging section leading to athroat sized to provide said restriction.

In one form of the invention the flow manipulator includes a divergingsection immediately downstream of the throttling duct, configured toinduce a shock wave in the mixture passing through the diverging sectionin use and provide said high energy dissipation rate.

In another form of the invention the throttling duct is arranged to openabruptly into a conduit extending within the separation cell, saidconduit having one or more openings in the separation cell adjacent thethrottling duct through which liquid is entrained in use from theseparation cell into the conduit.

Preferably the throttling duct and conduit are configured such thatunder desired operating conditions a shock wave is formed downstream ofthe throttling duct providing said high energy dissipation rate in thevicinity of the openings in the conduit.

In one configuration of the apparatus according to the invention thecolumn is located with its lower end within the separation cell, andwherein a plurality of said throttling ducts are provided orientatedradially outwardly adjacent the lower end of the column.

In another configuration the column is located with its lower end withinthe separation cell, and wherein the throttling duct is orientatedsubstantially downwardly at the lower end of the column and providedwith an impingement plate positioned substantially horizontally belowthe throttling duct, spaced therefrom so as to provide said flowmanipulator inducing said high energy dissipation rate within themixture passing therethrough.

Preferably the impingement plate comprises a lower circular disc alignedwith and spaced from an upper circular disc having a central holetherethrough arranged to receive mixture issuing from the throttlingduct, such that in combination with the diameter of the discs and theoperating pressure and velocity within the throttling duct, sonic flowconditions exist in use in or downstream of the throat in the throttlingduct.

In one form of the invention the lower disc is spaced a fixed distancefrom the upper disc, said distance being determined to provide saidsonic flow conditions.

In another form the lower disc is free to move in a vertical directionrelative to the upper disc, allowing the lower disc to come to a stableequilibrium in use, forming said sonic flow conditions.

In another form at least one of the upper and lower discs is flexibleand able to adapt to a shape dictated by pressure developed in the flowbetween the discs in use.

Preferably the lower plate is flexible and wherein the lower plate isprovided with a central solid wear resistant zone located a fixeddistance below the outlet from the throttling duct.

Preferably the mixture directing device comprises a draft tube in theform of a substantially vertical shroud located within the separationcell and arranged to direct the flow of mixture from the flowmanipulator into the separation cell.

In one form the shroud is open at both the upper and lower ends andpositioned to induce flow of liquid therethrough in a generally upwarddirection in use such that liquid within the lower part of theseparation cell is induced to flow upwardly through the shroud, joiningthe mixture issuing into the shroud from the flow manipulator.

Preferably the lower end of the shroud is restricted in size to controlthe flow rate of liquid passing into the shroud from the separationcell.

In one form the shroud is substantially constant in cross-section overthe majority of its length.

In another form the shroud is tapered outwardly and upwardly having agreater opening at the upper end than the lower end.

In yet another embodiment of the invention the shroud has a closed lowerend.

Preferably the impingement plate is located at the closed lower end ofthe shroud.

Preferably the relationship between the throttling duct, the impingementplate and the shroud is such as to form said flow manipulator causing arapidly rotating toroidal vortex within the lower end of the shroud andinducing said high energy dissipation rate within the mixture.

Preferably the relationship between the throttling duct, the impingementplate and the shroud is such as to form an expanded fluidized bed withinthe shroud when the apparatus is operated at desired parameters.

In a further aspect, the present invention provides a method ofcontacting bubbles and particles in a flotation separation system, saidmethod including the steps of:

providing apparatus including: a contactor arranged to receive underpressure a supply of feed slurry incorporating particles suspended in aliquid and a supply of gas, mixing means within the contactor arrangedto mix the slurry with the air forming a gas-liquid bubbly two-phasemixture, an outlet from the contactor configured to provide arestriction to the flow of mixture therethrough and maintain the mixturewithin the contactor under pressure, a flow manipulator downstream fromthe outlet configured to induce a high energy dissipation rate withinthe mixture passing therethrough, and a separation cell arranged toreceive mixture from the flow manipulator and allow bubbles withattached particles to rise to the surface of liquid within the cell;

and feeding slurry and gas into the contactor at feed rates andpressures determined to form said gas-liquid bubbly two-phase mixtureand force the mixture through said flow manipulator at a rate thatinduces said high energy dissipation rate within the mixture reducingthe size of the bubbles within the mixture and bringing those bubblesinto intimate contact with particles in the mixture.

Preferably the method includes the step of feeding the mixture from theflow manipulator into a mixture directing device within the separationcell.

Preferably the mixture is fed into the mixture directing device in amanner that, in combination with the shape of the mixture directingdevice, reduces turbulence within the mixture.

In one form the mixture directing device is a draft tube in the form ofa substantially vertical shroud arranged to direct the flow of mixtureupwardly into the separation cell.

Preferably the slurry is conditioned with collectors and frotherreagents prior to being fed into the contactor.

Preferably the collectors and frother reagents are selected to renderthe particles hydrophobic and able to form strong bonds with thebubbles.

In one use of the method the particles comprise minerals and theflotation separation system is operated to separate the minerals fromgangue or other contaminants. A typical example is the separation ofcoal particles from gangue.

In an alternative use of the method the feed slurry contains particlesof an organic nature and the flotation separation system is operated toremove those particles from the liquid.

In yet another use the particles are metal particles such as aluminiumparticles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view of a flotation device showing agas-liquid contactor, a flow restrictor and a riser pipe to direct theflow downstream of the restrictor according to the present invention;

FIG. 2 is a schematic view of the flow restriction device shown in FIG.1.

FIG. 3 is a schematic side view of the apparatus shown in FIG. 1 usingan alternative flow restriction device;

FIG. 4 is a schematic side view of an alternative gas-liquid contactorand riser pipe according to the invention;

FIG. 5( a) is an enlarged side view of the flow restriction shown inFIG. 3 and FIG. 4.

FIG. 5( b) is an enlarged plan view in the plane A-A in FIG. 4 showingthe disposition of the flow restrictions shown in FIG. 4 and FIG. 5( a);

FIG. 6( a) is an enlarged side view of an alternative restriction at theexit from the gas-liquid contactor and directing the discharge from therestriction in the radial direction;

FIG. 6( b) is an enlarged plan view of the restriction and radial flowdevice shown in FIG. 6( a);

FIG. 7 is an enlarged schematic side view of the restriction andalternative radial flow device shown in FIG. 6( a);

FIG. 8 shows a schematic side view of an alternative flow restrictor andapparatus to direct the downstream flow in a radial and then avertically-upwards direction;

FIG. 9 shows an alternative gas-liquid contactor, pressure reducingrestrictor and flow distribution means;

FIG. 10 shows a further alternative gas-liquid contactor andpressure-reducing means and conduit to direct the resulting gas-liquidmixture to the froth layer in a flotation column.

FIG. 11 shows the recovery of particles of various sizes subjected toflotation in a device according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

A first preferred embodiment of an intensive flotation column flotationcell according to the invention is shown in FIG. 1. The liquid feedcontaining the particles to be separated by flotation is prepared orconditioned with appropriate collectors and frother reagents prior toentry to the column, so that the values are hydrophobic and will be ableto form strong bonds with bubbles. The feed to the column enters at theinlet 10 and flows through the pre-mixing device 11 where it mixes withair which enters at 12. In this embodiment the gas is premixed with theliquid in a plunging jet apparatus prior to introduction to a pressurereducing means. The feed liquid enters a converging section 13 forming anozzle in which the liquid is accelerated to form a plunging jet 14 ofrelatively high velocity. A pressurised gas stream enters through theside arm 15, and is entrained into the high speed jet 14 to form agas-liquid mixture in which the bubbles are typically less than 0.5 nm nin diameter, in a conduit 16. The bubbly two-phase flow travelsvertically downwards to the bend 17 where it changes direction, andenters a throttling duct 18 which has the form of a converging-divergingchannel. Preferably the velocity of the gas-liquid mixture in the throatof the converging-diverging channel exceeds the speed of sound, when theflow is said to be “choked”. The flow becomes choked when the ratio ofthe absolute pressure upstream of the throat to the absolute pressuredownstream of the throat exceeds a critical value. When the pressureratio is above the critical value, the flow downstream of the throatbecomes supersonic, and a shock wave forms in the diverging section,which involves a large pressure rise over a very small physicaldistance, of the order of 3 to 5 mm. The small bubbles in the gas-liquidmixture are rendered even smaller by being forced through the shockwave, where they are brought into intimate contact with the hydrophobicparticles in the suspension to form bubble-particle aggregates. Theemulsion of fine bubbles and adhering particles then passes through theconnecting conduit 19 to a shroud in the form of a draft tube or riser20, before discharging into the flotation tank or column 21. The columncontains liquid whose upper surface 22 is maintained at a particularlevel by means not shown. The bubbles disengage from the liquid and risethrough the froth-liquid interface 22, carrying the hydrophobicparticles into the froth 23, which discharges over a lip 24 into alaunder 25 and thence out of flotation vessel through an exit conduit26. The liquid flows downwards to the base of the cell 21, and leavesthrough the exit pipe 27, and a valve 28 that is used to control thelevel of liquid in the cell.

Because the density of the gas-liquid mixture leaving the restrictivethroat 18 is less than that of the contents of the column 21, which isessentially that of gas-free liquid, an upwards convective flow isestablished through the draft tube 20. Liquid from the column is drawninto the base of the draft tube and is brought into contact with bubblesthat have been generated in the plunging jet contactor 16 and the chokedflow device 18 in combination. Thus a proportion of the particles thatmay not have made contact with bubbles when first entering the vesselthrough the contacting system, or which may have detached from the frothlayer 23 and fallen back into the liquid in the flotation vessel 21,will have an additional opportunity to become attached to bubbles and becarried by them into the froth layer. It has been found that if thedraft tube 20 is open-ended at its upper and lower extremities, theratio of the flowrate of recirculating liquid to that of the incomingfeed liquid, which is termed the internal recycle ratio, is quite large,of order 4 to 6. Such flowrates give rise to highly energetic flowswithin the cell 21, and a buoyant plume rises from the upper open end ofthe draft tube 21 whose velocity is so high that it can be disruptive tothe froth layer and lead to an increase in drop-back of particles fromthe froth. Accordingly it has been found to be advantageous toincorporate an entry tube 29, which restricts the internal recycle ratioto a value preferably between 2 and 3. The height/diameter ratio of thedraft tube 20 and the inlet pipe 29 are each preferably in the range 2to 5. The centreline of the horizontal conduit 19 should intersect withthe axis of the draft tube 20 at a height approximately equal to 1.5times the diameter of the conduit 19 above the lowest extremity of thesaid draft tube.

In this embodiment preferably the plunging jet contactor is mounted sothat the jet is directed vertically downwards. The cross-sectional areaof the plunging jet contactor 16 in a plane normal to the axis should besuch that the downward superficial velocity of the liquid is above theterminal velocity of the largest bubbles that are likely to form in thecontactor, and it has been found that an appropriate velocity is in therange 0.3 to 1 m/s. It is convenient to make the cross-sectional area ofthe inlet and outlet of the converging-diverging throttle 18 and thetransfer conduit 19, to be the same as that of the contactor 16. Thecross-sectional area of the draft tube 20 should be not less than thatof the contactor 16, and should preferably in the range 2 to 4 timessaid area. The area of the entry pipe 29 should be in the range 0.1 to0.5 of the cross-sectional area of the draft tube 20.

The area of the throat is chosen with advantage so that the gas-liquidmixture formed in the contactor 16 attains the speed of sound there. Ifthe sonic velocity is exceeded, a shockwave forms downstream of thethroat, which has an effect on the size of the bubbles in the flow. FIG.2 shows a shock wave bubble generator according to the present inventionin greater detail. In FIG. 2, the device 18 comprises a conduit 31, aconverging section 32, a throat 33 in which the walls are essentiallyparallel, a relatively slowly diverging section 34 and a deliveryconduit 19, the walls of which may conveniently be parallel. Agas-liquid mixture, preferably well-mixed so that the bubbles arealready finely divided, enters the entry conduit 31. Preferably thevelocity of the gas-liquid mixture in the region upstream of the throatis sub-sonic; the velocity of the gas-liquid mixture in the throat 33reaches the speed of sound in the mixture at that point; a region offlow exists downstream of the throat 33 in which the gas-liquid mixtureaccelerates and reaches supersonic velocities; a shock wave 35 isproduced in the slowly-diverging section 34; the flow reverts to asubsonic condition in the region immediately downstream of the shockwave and the velocity of the gas-liquid mixture is further reduced inthe diverging region downstream of the shock wave. The mixture leavesthe device at a convenient subsonic final velocity at the exit 36 to theconduit.

The way in which small bubbles are produced in the apparatus describedcan be explained with reference to the changes in the pressure in thetwo-phase mixture. In the entry region 31 the pressure is constant inthe gas and liquid phases, and is denoted the “upstream pressure.” Whenthe mixture accelerates in the converging region 32, the pressurereduces according to well-known laws of fluid flow, so the bubbles inthe mixture become larger. In the throat 33, at a critical value of theupstream pressure, the gas-liquid mixture reaches the speed of sound inthe mixture. If the upstream pressure is sufficiently large, the fluidcontinues to accelerate downstream of the throat 33, and the pressurecontinues to fall, so that the bubbles continue to increase in volume.At a certain point in the diverging region, a shock wave 35 occurs,across which there is a catastrophic change in the flow, and thepressure rises from a small value ahead of the shock to a large valuedownstream. Because of the rapid pressure change, the large bubblesahead of the shock break up in a violent fashion, to form very smallbubbles, typically less than half the size of the bubbles in the flow inthe entrance duct 31. It has been found that the thickness of the shockwave in the flow direction is relatively small, being in the range 3 to5 mm typically. It will be appreciated that a purpose of this inventionto bring about contact between hydrophobic particles and small bubbles.The chaotic motions that occur within the shock wave have the effect notonly of breaking up the bubbles, but also of freshly creating a verylarge interfacial gas-liquid area in a high-energy, intensively-mixedzone within the shock wave and downstream of it. The combination of verysmall bubbles and high-energy mixing has the effect of bringing aboutinstant contact between the bubbles and the hydrophobic particles.

The cross-sectional area to achieve a sonic velocity in the throat shownin FIG. 2, can be calculated from an equation that has beenexperimentally verified (Sandhu, N., Jameson, G. J. An experimentalstudy of choked foam flows in a converging-diverging nozzle,International Journal of Multiphase Flow (1979), 5, 39). The equation ispresented here in the form:

$\begin{matrix}{{1 - {\left( \frac{P_{3}\delta_{3}}{P_{0}} \right)\frac{1}{\delta_{t}}} - {\left( \frac{P_{3}\delta_{3}}{p_{0}} \right){\ln \left\lbrack {\left( \frac{P_{3}\delta_{3}}{P_{0}} \right)\frac{1}{\delta_{t}}} \right\rbrack}}} = {\frac{1}{2}{\left( \frac{P_{3}\delta_{3}}{P_{0}} \right)\left\lbrack {1 + \frac{1}{\delta_{t}}} \right\rbrack}^{2}}} & (1)\end{matrix}$

where P₀ is the pressure in the conduit 31 upstream of the throat; δ_(t)is the gas/liquid volume ratio in the throat 33; and δ₃,P₃ arerespectively the gas/liquid volume ratio and the pressure in thedischarge conduit 19. (All pressures are in units of Pascals absolute).The gas/liquid ratio in the throat δ_(t) can be represented as thedimensionless liquid flowrate:

$\begin{matrix}{{\frac{1}{\delta_{t}} = \frac{Q_{L}/A_{t}}{\left( {P_{3}{\delta_{3}/\rho_{L}}} \right)^{0.5}}},} & (2)\end{matrix}$

where Q_(L) is the volumetric flowrate of liquid (m³/s); ρ_(L) is thedensity of the liquid (kg/m³) and A_(t) is the flow area in the throat33 (m²). Thus if the downstream conditions, i.e. the pressure and thegas/liquid volume ratio in the aerated mixture entering the flotationcell, are known, it is possible to solve equation 1 to find the criticalvalue of the upstream pressure P₀ for the velocity in the throat 33 toreach the speed of sound and hence for the flow to be choked. Anyincrease in pressure above the critical value will lead to the formationof a shock wave downstream of the throat.

It is not possible to find analytic solutions to Equation 1. However, ithas been found that the following equation, which can be solved easily,is an excellent representation of Equation 1 for values of thedimensionless liquid flowrate (1/at ) less than 2.5:

$\begin{matrix}{\frac{p_{0}}{p_{3}\delta_{3}} = {{0.5516\left( \frac{1}{\delta_{t}} \right)^{2}} + {1.655{\left( \frac{1}{\delta_{t}} \right).}}}} & (3)\end{matrix}$

The rate of capture of particles by flotation can be enhanced byincreasing the shear rate, or rate of dissipation of energy, in thevicinity of the particles and the bubbles. The shear rate isproportional to the square root of the rate of energy dissipation. Inthe embodiment shown in FIG. 1, the energy dissipation rate downstreamof the throat 33 is high because much of the energy stored in thepressurised feed and gas entering the throat is dissipated in theshockwave 35, which is typically 3 to 5 mm in thickness. It can becalculated for example that the energy release rate in the shockwave isof the order of 20,000 kW/cubic metre of shockwave. For particle-bubblecontacting purposes however it can be advantageous to release the sameamount of energy into a larger volume of liquid as in the embodimentshown in FIG. 3, which provides a longer time for the particles andbubbles to come into contact. In conventional mechanical flotationmachines, the energy dissipation rate is generally in the range 2 to 3kW of power per cubic metre of liquid in the flotation cell. Contactbetween bubbles and particles takes place in the volume enclosed by theimpeller and stator in the cell, which is typically of the order of 5 to10 percent of the volume of the cell. Accordingly the effectivedissipation rate in mechanical cells is of the order of 50 kW per cubicmetre of active volume. In flotation columns, the energy dissipationrate is much less. It is an aim of the present invention to provide anactive contacting environment downstream of the flow restriction inwhich the energy dissipation rate is at least as high as found inmechanical flotation cells. In the embodiment shown in FIG. 3, thedissipation rate in the volume immediately downstream of the throat 33is of the order of 100 to 150 kW/m3.

The embodiment shown in FIG. 1 has the advantage that, through the useof the divergent diffuser downstream of the throat, the maximum amountof mechanical energy in the feed to the choke can be recovered, whichcan be an important consideration when running costs are important.However, in some cases, energy costs are outweighed by other factors,especially where it is possible to increase the recovery of valuableparticles. A further embodiment is shown in FIG. 3, in which energyrecovery is reduced but where the mechanical energy that is lost is usedto improve the contacting between the incoming feed liquid and theliquid in the flotation column. The slowly-diverging diffuser 34 isdispensed with. The mixture of gas bubbles from the aerating contactor11 enters through the conduit 31, and accelerates in the convergingchannel 32 to a throat 33 which discharges directly into a duct 19;there is no slowly-diverging diffuser 34. The short form of theconstriction is denoted 37. The critical pressure for the attainment ofsonic flow in the throat is predicted by equations (1) and (3) asbefore. If the critical pressure is exceeded, shockwaves are formeddownstream of the throat, but they are not necessarily bounded by asolid wall. The flow issuing from the throat takes the form of agas-liquid wall jet with considerable velocity, of the order of 20 m/s.The throat discharges directly into the conduit 19, in the walls ofwhich openings 41 have been formed, through which liquid is entrainedfrom the flotation column 21. Particles that may have escaped capture inthe first pass of the feed liquid into the combined aerating contactor11 and the restricting throat 33, have an additional opportunity to becaptured by bubbles freshly entering the column through the throat 33.This embodiment is particularly favourable for the capture of ultrafineparticles, because of the creation of a high-shear environment with ahigh gas/liquid ratio, in the mixing zone downstream of the jet issuingfrom the throat 33. One or more openings 41 should be provided. Theopenings are conveniently located equi-spaced around the circumferenceof the conduit 19, at a position downstream of the throat equal to 0.5to 2 times the throat diameter. The total flow area of the openings 41should be approximately equal to the cross-sectional area of the conduit19.

In all the embodiments disclosed here, the throat length shouldpreferably be in the range 0 to 3 times the throat diameter.

An advantage of using the converging-diverging nozzle shown in FIG. 2and the truncated form shown in FIG. 3, is that high ratios of gas tofeed liquid can be dispersed into small bubbles in such devices,especially when operated in choked conditions where shockwaves formdownstream of the nozzle. In the case of a nozzle discharging into aflotation cell at essentially atmospheric pressure, for gas:liquidratios of 0.5 to 4 at the same pressure, the pressure upstream of thechoke is typically 1.7 to 4 times the downstream pressure, implying thatthe gas:liquid ratio within the contactor 16 is in the range 0.58 to0.25. The gas:liquid ratio has a strong effect on bubble generation andgenerally, finer bubbles can be formed when the volume ratio of gas toliquid is small, because of the reduction in the rate of coalescence ofbubbles subsequent to formation. High gas:liquid ratio dispersions ofbubbles in the flotation slurry are highly desirable, because they leadto high values of the specific surface area of bubbles, which leads inturn to higher carrying capacity or production from the flotation deviceas a whole. Accordingly, in the embodiments described, it is convenientto operate with gas:liquid ratios in the flotation column between 0.5and 4.

A further embodiment is shown in FIG. 4, in which the gas-liquidcontactor 16 is mounted by means not shown essentially co-axially withthe flotation column 21. Suitably conditioned feed liquid is introducedthrough the inlet pipe at 10 which has a converging section 11 in whichthe liquid is accelerated to form a plunging jet 14 of relatively highvelocity. A gas stream under pressure enters through a side arm 15, andis entrained into the high speed jet 14 to form a gas-liquid mixture inthe contactor 16 in which the bubbles are substantially less than 1 mmin diameter. The bubbly mixture travels downwards to the lower end ofthe downcomer, where it passes into a discharge nozzle 37 shown in moredetail in FIGS. 5( a) and 5(b). Each exit nozzle 37 communicates withthe liquid inside a flotation column 21. The liquid flows downwards tothe base of the cell 21, and leaves through the exit pipe 27, and avalve 28 that is used to control the level of liquid in the cell. Theupper lip 24 of the vessel 21 forms an overflow weir for froth 23 whichis collected in a launder 25 and is drained away through an outlet 26.

In operation, the contactor 16 is filled with a dense foam that travelsdownwards to discharge through one or more discharge nozzles 37. Thebubbles in the mixture discharged from the contactor mix with the liquidin the containing vessel 21 and disengage from it, rising to the top ofthe vessel to form the froth layer 23. The level of liquid in the outervessel or container is maintained by the valve 28 or other means, at alevel 22. Air is introduced through the entry port 12, at a pressure andflowrate so that the downcomer 16 fills with a dense foam that isagitated by the entering jet of liquid 14, that carries the particulatematerial to be collected by the bubbles. The turbulent mixing created bythe kinetic energy in the plunging jet is a highly favourableenvironment for the capture of particles by the bubbles in the densefoam. Because of the violent and turbulent nature of the plunging jetthe particles in the feed liquid are brought into intimate contact withthe bubbles, thus providing a favourable environment for the collectionof the hydrophobic particles by the bubbles. Because of the flowrestriction brought about by the discharge nozzle 37, the pressure inthe downcomer 16 is well above the ambient pressure in the containingvessel 21 at the discharge end of the nozzle 37. The small bubbles inthe gas-liquid mixture are rendered even smaller by being forced throughthe nozzle, where they are brought into further intimate contact withthe hydrophobic particles in the suspension to form bubble-particleaggregates. The pressure of the liquid feed and the air supply are suchas to be able to maintain the flow of gas and liquid through thedischarge nozzles 37.

The gas-liquid mixture that discharges from the shortened nozzle 37,which consists only of the converging section 37 and the parallel-walledthroat 33, does so at a considerable velocity, and the momentum in theflow can be utilised further, to increase the overall efficiency of theflotation system. Thus it has been found advantageous to incorporate aninternal draft tube 20, which surrounds the lower end of the contactor16. Because the average density of the gas-liquid mixture beingdischarged into the draft tube is lower than the density of the liquidin the vessel 21, it tends to rise in the vertical direction, and acirculating pattern is created. Liquid from the vessel is drawn into theentry tube 29 which would otherwise be passing directly out of thetailings exit pipe 28, so the incorporation of the draft tube leads tothe further exposure of the particles in the recirculated liquid to thebubbles discharging from the nozzle(s) 37, thereby leading to furtheropportunities for capturing some particles that would otherwise pass outof the vessel

In relation to the embodiment shown in FIG. 4, FIG. 5( a) shows anelevation view of a discharge nozzle 37, and FIG. 5( b) shows a planview of an embodiment comprising three nozzles 37 equi-spaced around theperiphery of the contactor 16. It will be appreciated that one or morenozzles could be used, in which case the total flow area of the throats33 of the individual nozzles should be used in the calculation of theupstream pressure P₀ in the contactor 16.

FIG. 6( a) shows an alternative embodiment of the pressure restrictionand dispersion means for use at the termination of the initial contactor16. A mixture of gas bubbles and liquid slurry formed in the contactorenters a converging conduit 32 of a truncated choke 37 and passes to athroat 33 from which it leaves through a radial diffuser in the spacebetween an upper circular disc 43 and a lower circular disc 44. Thediscs 43, 44 that define the radial passageway of the disperser aresubstantially horizontal.

In the embodiment shown in FIG. 6( a), the two circular discs 43,44 maybe held at a fixed distance apart, so that the flow passage between themis of constant vertical height. In this case, the velocity between thediscs decreases continuously with increasing radial distance from theaxis. If the velocity is sufficiently high, sonic flow conditions willexist in or downstream of the throat 33. Surprisingly, it has been foundadvantageous to mount the lower disc so that it is free to move in thevertical direction. Because of the changes in velocity within the spacebetween the discs, the pressure in said space is substantially less thanthe pressure at the end of the radial channel, and hence, is less thanthe pressure in the liquid external to the radial disperser. A largeforce is thus induced that tends to push the two discs together. If thelower disc is free to move, it will come to a stable equilibrium at acertain distance from the upper disc. Observation suggests that in thiscase the speed of sound is reached in the gas-liquid mixture when itreaches the outermost region 46 of the radial passage between the discs.

It is a property of the converging flow in the radial channel 45, thatthe suction induced in said channel decreases as the separation distanceh increases. This observation gives a significant practical advantage tothe case where the lower disc is free to move in the vertical direction,in that if the space between the discs becomes blocked by a largeparticle, the pressure in the radial channel 45 will increase and willforce the lower disc 44 to move away from the upper disc 43, therebyreleasing the large particle which is swept away in the flow.

In another embodiment shown in FIG. 7, upper and lower discs 43 and 44are provided that are flexible and compliant to the flow conditions.Thus they can adapt to a shape that is dictated by the pressuredeveloped in the flow within the radial passage 45. It has been foundthat in such a case, the spacing between the two circular discs at theexit 46 can be very small, leading to high velocities in the gas-liquidstream leaving the periphery of the radial disperser. The smallthickness of the gas-liquid mixture at the exit is conducive to theproduction of very small bubbles. In such an apparatus, one or both ofthe opposing discs can be flexible. An impingement plate 47 is provided,to absorb the stagnation pressure of the impinging liquid jet emanatingfrom the throat 33. It is preferred that both the converging nozzle 37and the impingement plate 47 be of a wear-resistant substantially solidmaterial. It is preferable for the impingement plate to be restrained bya means not shown, at a fixed distance from the throat 33, and on thesame vertical axis. This embodiment is particularly useful in a feedstream containing coarse particles. Because of the flexible propertiesof the material used for either or both of the upper and lower discs, itis not necessary for a complete disc to move in order to release aparticle that may have become lodged in the radial channel 45—all thatis required is for one of the discs to distort locally, in the region ofthe particle, for the latter to be released, thereby unblocking thechannel.

A further embodiment is shown in FIG. 8, which depicts a restrictivethroat at the lower extremity of a first contacting device 16. Themixture of fine bubbles and slurry passes through the throat 33 underpressure, and strikes the impingement plate 54 at high velocity,spreading out in the radial direction. Because of the high momentum inthe jet, liquid is entrained, and the jet expands as it travels radiallyoutwards. The draft tube 20 restricts the outwards radial motion of thejet, and a toroidal vortex 55 is formed. The average rate of shear inthe toroid is very high, and an environment that is very favourable tothe break-up of bubbles and the contacting of bubbles and particlesexists. Remarkably, it has been found that the gas fraction in theregion surrounding the vortex can be maintained at values as high as0.65, approaching the maximum packing fraction of spheres. The high gasfraction also leads to rapid contact of bubbles and hydrophobicparticles, especially for larger particles, because the distance betweenbubbles is smaller than the size of the particles. Flotation efficiencyis further improved by the buoyancy-induced flow created in the drafttube 20, which permits some of the liquid that has previously enteredthrough the pressure restricting throat 33, which may contain particlesthat have dropped out of the froth, to be recycled.

FIG. 9 shows a further embodiment in which the flotation device consistsof a separation vessel 21 which can be conveniently cylindrical in form,with a conical bottom 59, a froth overflow lip 24 at the upper end ofthe cylindrical vessel, which is surrounded by a launder 25 fitted withan outlet 26 for the removal of froth product from the device. At thelower end of the separation vessel is a conduit 27 for the discharge oftailings under the control of a valve 28. The level of liquid in thevessel is maintained at a suitable level by means not shown. Liquid feedunder pressure enters the separation apparatus through a nozzle 61. Thefeed is a suspension in water of particles to be treated by frothflotation, which have been suitably conditioned by reagents and frothersas appropriate. At the exit of the nozzle 61, the feed forms a liquidjet which enters a first chamber 62 and mixes with air that has beenintroduced under pressure through an entry pipe 15. Air is entrainedthrough the turbulent mixing action of the jet, and is dispersed intosmall bubbles in the liquid, which travels downwards through the firstchamber 62 to a second nozzle 64. In the second nozzle 64, the bubblyflow is forced under pressure to reach a velocity that is approximatelyequal to the speed of sound in the mixture. Under such conditions thereare abrupt changes in pressure downstream of the nozzle exit 65, suchthat the bubbles in the flow are broken into smaller gas fragments. Itis not essential that the sonic velocity of the mixture is reached inthe nozzle 64; alternatively the conditions in the second nozzle aresuch as to provide a positive backpressure in the first chamber 62, andreliance is placed on the shearing action of the jet that issues fromthe second nozzle 65 to break up the bubbles within it as it mixes withthe downstream fluid.

The exit stream from the second nozzle enters a second chamber 66, whichis fitted with appropriately-placed ports 67, through which fluid can bedrawn, to dilute the liquid content in the jet emanating from the secondnozzle 64. The combined flow of gas-liquid mixture from the nozzle 64,and recirculating flow through the entry ports 67, passes downwardsthrough the second chamber 66, to discharge through the opening 68.

Surrounding the second chamber 66 and co-axial with it is a draft tube69 that is conveniently of conical shape. The combined flow leaving thesecond chamber 66 contains both gas and liquid, and accordingly is oflower mean density than the liquid in the flotation vessel 21, so itrises under gravity in the annular space between the chamber 66 and thedraft tube 69, filling the said annular space with a bubbly mixture.Liquid from the lower part of the separation vessel 21 is drawn throughthe port 70 at the lower extremity of the draft tube 69.

The two-phase gas-liquid mixture rising out of the open upper end 71 ofthe draft tube enters the upper part of the separation vessel 21, andthe gas bubbles rise upwards and separate from the liquid to form afroth layer 23. The froth rises upwards and discharges over the lip 24into the launder 25 and out of the vessel through the exit pipe 26. Thetailings, from which the floatable material has substantially beenremoved, pass out through the pipe 27. This embodiment is particularlyappropriate for the recovery of coarse particles, because the conicaldraft tube 69 can be of such dimensions and placed in such a way thatdistance between the top of the said draft tube and the froth-liquidinterface 22 can be minimised. The tapered shape of the conical drafttube permits the upward velocity of the mixture of liquid andparticle-laden bubbles to diminish with height generating a quiescentflow leaving the upper exit of the draft tube 69, thereby enhancing theprobability of retention of coarse particles by the bubbles.

A further embodiment is shown in FIG. 10 in which pressurised aeratedslurry from a first chamber discharges into a second contacting chamberas a high-speed jet, of velocity typically in the range 10 to 20 m/s.The contents of the base of the second chamber are vigorously agitatedby the energy in the jet providing an environment that is particularlyfavourable for further reducing the size of the bubbles and forcapturing hydrophobic particles in the feed. The gas fraction in thelower parts of the second chamber may be as high as 0.5 to 0.6, valuesthat are typical of a dense liquid foam, and particularly useful forcapturing coarse particles. The height of the second chamber is suchthat when the gas-liquid mixture nears the top, the flow is relativelyquiescent. The bubbles continue to rise into the froth layer, while thewaste particles are carried out of the vessel. Particles that drop backfrom the froth fall directly into the second chamber under gravity wherethey have an additional opportunity to attach to fresh rising bubbles.

In the embodiment shown in FIG. 10, liquid feed under pressure entersthe separation apparatus through a nozzle 81. The feed is a suspensionin water of particles to be treated by froth flotation, which have beensuitably conditioned by reagents and frothers as appropriate. At theexit of the nozzle 81, the feed forms a liquid jet 14 which enters afirst chamber 16 and mixes with air that has been introduced underpressure through an entry pipe 15. Air is entrained through theturbulent mixing action of the jet, and is dispersed into small bubblesin the liquid, which travels downwards through the first chamber 16 to asecond nozzle 83, from which it issues under pressure through the throat84. Bubbles that have been formed in the first chamber 16 are furtherreduced in size by the pressure changes as they pass through the nozzle83, and by the high-shear environment downstream of the nozzle. The exitstream from the second nozzle enters a second chamber 85, which isconveniently cylindrical in shape, and of a diameter much larger thanthat of the first chamber 16. The high-speed gas-liquid jet that issuesfrom the nozzle 83 is directed downwards against an impingement plate 86that is constructed of a high-wear material, and is deflected so as toflow radially outwards to the conical base 87 of the second chamber. Inthe base of the second chamber, the gas-liquid mixture is highlyagitated by the energy in the incoming jet, and forms a rapidly-rotatingtoroidal vortex 55, in which the size of the bubbles is reduced by thehigh-shear conditions, which are also favourable to high rates ofcontact between bubbles and particles in the liquid. As the mixturerises, the general level of turbulence reduces and the flow at the topof the second chamber 85 is relatively uniform.

The two-phase gas-liquid mixture rising out of the open upper end 88 ofthe second chamber 85 enters the upper part of the separation vessel 21,and the gas bubbles rise upwards and separate from the liquid to form afroth layer 23. The froth rises upwards and discharges over the lip 24into the launder 25 and out of the vessel through the exit pipe 26. Thetailings, from which the floatable material has substantially beenremoved, pass out through the pipe 27.

It is advantageous to be able to control the liquid velocity rising inthe riser conduit that forms the second chamber 85, especially when theparticles are so large that their terminal velocity is greater than theliquid vertical velocity in the riser. In the embodiments shown in FIGS.8 and 9, it is difficult precisely to control the velocity in the drafttube, which is a function not only of the gas fraction in the feed fluidbut also the solids fraction in the feed and in the liquid external tothe draft tube. In the embodiment shown in FIG. 10, the riser has aclosed base, and the superficial rise velocity of the liquid across theexit plane 88 is related simply to the liquid flowrate through thethroat 84 and the cross-sectional area perpendicular to the flow at 88.The feed does not contain individual particles at infinite dilution. Inpractice, the feed consists of a suspension of particles at a finitevolume fraction, and hence the terminal velocity of individual particlesis less than the terminal velocity at infinite dilution because of thephenomenon known as hindered settling. Thus it is not necessary for thevertical velocity in the riser 85 to exceed the terminal velocity ofindividual particles, in order for such particles to be carried upwardsand out of the riser; all that is required is to maintain a velocitythat exceeds the hindered settling velocity, so that the suspensionforms an expanded fluidised bed. Accordingly, in the present embodimentthe device should be sized to maintain the hydrophobic and hydrophilicparticles in the feed in a suspended state in the second chamber 85. Thehydrophobic particles attached to bubbles will rise out of the liquidand into the froth layer, while other particles will flow with theliquid down the annular gap 89 between the column 21 and the outer wallof the second chamber 85. In practice it is found that some of thecoarse hydrophobic particles that are carried into the froth,subsequently disengage from bubbles and drop back into the vessel 21, asa result of bubble coalescence in the froth. In the embodiment shown inFIG. 10, the majority of such particles will fall back into the secondchamber 85 where they will be captured by bubbles newly entering thesystem, and carried once more into the froth.

The invention is described in terms applicable to the separation ofminerals in which ore is finely crushed to form a slurry or suspensionof particles in water, and the slurry is conditioned with collector andfrother to make the mineral species that is to be recovered by flotationhydrophobic or non-wetting, while the non-wetting or hydrophilic speciesthat are to remain in the suspension and are discharged from theflotation vessel as tailings. An example of this is the separation offine coal particles from the surrounding gangue in a mining operation.

However the invention will also apply to systems in which the particlesare of an organic native and typically of biological or non-metallicorigin such as algae, printing ink, dairy fat or other liquidparticulate systems. The invention will also apply to systems in whichall the particles are to be removed in the froth, there being norequirement to separate the components of the particles in the feedliquid on the basis of their hydrophobicity or lack thereof.

A further application is in the removal of metals such as aluminium fromsuspensions.

EXAMPLE

Samples of silica were subjected to flotation in an embodiment of theinvention according to FIG. 1. The silica had a top size of 48 micronsand half of the particles in the sample by mass had a particle sizebelow 7.9 microns. Dodecylamine was used as collector at 500 gm/tonne,and methyl isobutyl carbinol at a concentration of 20 ppm was used asfrother. The silica, at a concentration of 5% W/W was conditioned in afeed tank for ten minutes, before being pumped to the gas-liquidcontactor. In two separate runs the volume ratios of the air flowrate tothe flowrate of feed in the flotation column were 1:1 and 2:1respectively. The pressures upstream of the choking nozzle are shown inTable 1. The overall recovery was calculated from measurements of theflowrates of the feed, the product and the tailings, and the percentsolids in each flow. To correct for the presence of entrained materialin the concentrate, the amount of entrainment was estimated on theassumption that the water in the froth product contained silica at thesame concentration as in the tailings. Tests were conducted with adevice constructed according to the present invention and also forcomparison, an existing technology in the form of a conventional Jamesoncell was used. The results are shown in

TABLE 1 Pressure Air:feed upstream of Overall True Flotation volumetricconstriction, recovery recovery machine ratio kPa gauge % % Thisinvention 1:1  90 77 74 2:1 150 93 87 Existing 1:1 — 54 51 technology

The true recoveries were also calculated on a size-by-size basis, andthe results are shown in FIG. 11.

1-35. (canceled)
 36. Apparatus for contacting bubbles and particles in aflotation separation system, said apparatus comprising: a contactorarranged to receive under pressure a supply of feed slurry incorporatingparticles suspended in a liquid and a supply of gas, the contactor beingarranged to mix the slurry with the air forming a gas-liquid bubblytwo-phase mixture; an outlet from the contactor configured to provide arestriction to the flow of mixture therethrough and maintain the mixturewithin the contactor tinder pressure; a flow manipulator downstream fromthe outlet configured to induce a high energy dissipation rate withinthe mixture passing therethrough; and a separation cell arranged toreceive mixture from the flow manipulator and allow bubbles withattached particles to rise to the surface of liquid within the cell. 37.Apparatus for contacting bubbles and particles in a flotation separationsystem as claimed in claim 36, wherein the separation cell is providedwith a mixture-directing device arranged to receive the mixture from theflow manipulator and control the release of that mixture into the cell.38. Apparatus as claimed in claim 36 wherein the contactor comprises asubstantially vertical column arranged to receive the feed slurry underpressure into the top of the column.
 39. Apparatus as claimed in claim38 wherein the container incorporates mixing means comprising a nozzlearranged to form a downwardly plunging jet of feed slurry within thecolumn, and a gas inlet in the vicinity of the jet so formed such thatin use gas is entrained into the jet forming said gas-liquid bubblytwo-phase mixture.
 40. Apparatus as claimed in claim 38 wherein theoutlet from the contactor is configured to form at least one throttlingduct providing said restriction to the flow of mixture therethrough. 41.Apparatus as claimed in claim 40 wherein the throttling duct has aconverging section leading to a throat sized to provide saidrestriction.
 42. Apparatus as claimed in claim 40 wherein the flowmanipulator comprises a diverging section immediately downstream of thethrottling duct, configured to induce a shock wave in the mixturepassing through the diverging section in use and provide said highenergy dissipation rate.
 43. Apparatus as claimed in claim 40 whereinthe throttling duct is arranged to open abruptly into a conduitextending within the separation cell, said conduit having one or moreopenings in the separation cell adjacent the throttling duct throughwhich liquid is entrained in use from the separation cell into theconduit.
 44. Apparatus as claimed in claim 43 wherein the throttlingduct and conduit are configured such that under desired operatingconditions a shock wave is formed downstream of the throttling ductproviding said high energy dissipation rate in the vicinity of theopenings in the conduit.
 45. Apparatus as claimed in claim 40 whereinthe column is located with its lower end within the separation cell, andwherein a plurality of said throttling ducts are provided orientatedradially outwardly adjacent the lower end of the column.
 46. Apparatusas claimed in claim 40 wherein the column is located with its lower endwithin the separation cell, and wherein the throttling duct isorientated substantially downwardly at the lower end of the column andprovided with an impingement plate positioned substantially horizontallybelow the throttling duct, spaced therefrom so as to provide said flowmanipulator inducing said high energy dissipation rate within themixture passing therethrough.
 47. Apparatus as claimed in claim 46wherein the impingement plate comprises a lower circular disc alignedwith and spaced from an upper circular disc having a central holetherethrough arranged to receive mixture issuing from the throttling aduct, such that in combination with the diameter of the discs and theoperating pressure and velocity within the throttling duct, sonic flowconditions exist in use in use or downstream of the throat in thethrottling duct, and wherein the lower disc is spaced a fixed distancefrom the upper disc, said distance being determined to provide saidsonic flow conditions.
 48. Apparatus as claimed in claim 47 wherein thelower disc is free to move in a vertical direction relative to the upperdisc, allowing the lower disc to come to a stable equilibrium in use,forming said sonic flow conditions.
 49. Apparatus as claimed in claim 47wherein the lower disc is flexible and able to adapt to a shape dictatedby pressure developed in the flow between the discs in use, and whereinthe lower disc is provided with a central solid wear resistant zonelocated a fixed distance below the outlet from the throttling duct. 50.Apparatus as claimed in claim 37 wherein the mixture-directing devicecomprises a draft tube in the form of a substantially vertical shroudlocated within the separation cell and arranged to direct the flow ofmixture from the flow manipulator into the separation cell. 51.Apparatus as claimed in claim 51 wherein the shroud is open at both theupper and lower ends and positioned to induce flow of liquidtherethrough in a generally upward direction in use such that liquidwithin the lower part of the separation cell is induced to flow upwardlythrough the shroud, joining the mixture issuing into the shroud from theflow manipulator.
 52. Apparatus as claimed in claim 51 wherein the lowerend of the shroud is restricted in size to control the flow rate ofliquid passing into the shroud from the separation cell.
 53. A method ofcontacting bubbles and particles in a flotation separation system, saidmethod comprising the steps of: providing apparatus comprising: acontactor arranged to receive under pressure a supply of feed slurryincorporating particles suspended in a liquid and a supply of gas,mixing means within the contactor arranged to mix the slurry with theair forming a gas-liquid bubbly two-phase mixture, an outlet from thecontactor configured to provide a restriction to the flow of mixturetherethrough and maintain the mixture within the contactor underpressure, a flow manipulator downstream from the outlet configured toinduce a high energy dissipation rate within the mixture passingtherethrough, and a separation cell arranged to receive mixture from theflow manipulator and allow bubbles with attached particles to rise tothe surface of liquid within the cell; and feeding slurry and gas intothe contactor at feed rates and pressures determined to form saidgas-liquid bubbly two-phase mixture and force the mixture through saidflow manipulator at a rate that induces said high energy dissipationrate within the mixture reducing the size of the bubbles within themixture and bringing those bubbles into intimate contact with particlesin the mixture.
 54. A method as claimed in claim 53 comprising the stepof feeding the mixture from the flow manipulator into a mixturedirecting device within the separation cell in a manner that, incombination with the shape of the mixture directing device, reducesturbulence within the mixture.
 55. A method as claimed in claim 54wherein the mixture directing device is a draft tube in the form of asubstantially vertical shroud arranged to direct the flow of mixtureupwardly into the separation cell.